Stem replacement question

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I have a 120 mm 90 degree stem (quill) and would like a little less reach. I figured I could get a taller stem with a bit higher angle or an adjustable stem, but I'm having trouble identifying the correct size.

The specs from the bike say 24.5 mm insertion, 24.5 mm clamp. My understanding is that this means it is a 1 1/8" stem with a standard mtn bike clamp. I'm quite confused by the 1" vs 1 1/8" thing, and want to be sure I get the right size. I have not yet removed the existing stem to measure the opening, but how does the opening size relate to the stem size?

From your description, it's almost certainly a so-called "1-1/8" stem, given the "25.4 mm insertion" spec. But to be sure of the size, measure the outside diameter of the quill part of the stem with calipers. You don't need to remove the stem to do this If it's 22.2 mm (= 7/8" exactly), then you've got a 1" headset, if it's 25.4 mm (= 1" exactly), then it's a 1-1/8" headset.

The measurements are confusing because the headset size (1" or 1-1/8") doesn't actually refer to any exact measurement of anything

okay, no calipers, but I pulled the current stem and measured the diameter of the opening and got a max of 1" (assuming I didn't always get an exact cross through the center, so looking for a maximum measurement), so I'm going with the 1 1/8 inch replacement. If it doesn't fit I guess I'll know that its a 1"